The present invention pertains to an apparatus and methods for magnetic resonance imaging, also known as magnetic resonance imaging (MRI) and, in particular, to an apparatus and methods for nuclear magnetic resonance imaging (NMRI) for decreasing magnetic resonance (MR) data acquisition times, wherein magnetic resonance data is acquired in parallel using an array of receiver coils at least partially surrounding the object of interest, and the desired MRI image is then reconstructed in parallel.
In dynamic MRI applications, such as functional imaging, interventional imaging and cardiac imaging, there has long been a need in the art for methods and apparatus that provide high quality (e.g., high-resolution and signal-to-noise ratio) images. Conventional MRI imaging apparatus and methods, however, operate at speeds that are an order of magnitude slower than those which are currently deemed to be desirable. Some of these conventional methods are described in the Background section of U.S. Pat. No. 5,365,172 to Hrovat et al. for xe2x80x9cMethods and Apparatus for MRIxe2x80x9d, the disclosure of which is hereby incorporated by reference herein.
In an attempt to attain faster operating speeds, several so-called xe2x80x9cparallelxe2x80x9d encoding apparatus combinations and/or methods have been developed. These apparatus combinations and/or methods rely on the use of multiple receiver coils for the acquisition of magnetic resonance data and high-speed data processors for the reconstruction of the field of view with significantly smaller data sets.
Among the parallel imaging techniques described in the literature, the work of Kwiat et al. (xe2x80x9cA Decoupled Coil Detector Array for Fast Image Acquisition in Magnetic Resonance Imagingxe2x80x9d, Medical Physics, 18:251, 1991, the disclosure of which is hereby incorporated by reference herein) is significant. This work involved the investigation of methods for solving the inverse source problem on magnetic resonance signals received in multiple RF receiver coils. The technique proposed required the use of a number of RF coils equal to the number of pixels in the desired image. It also required that the sensitivity of the coils used be increased by an order of magnitude. Since these requirements are quite impractical in conventional magnetic resonance imaging (wherein the usual number of pixels in the image is on the order of 256xc3x97256), this technique has never been used successfully in a biological imaging experiment.
Other so-called xe2x80x9cparallelxe2x80x9d imaging techniques that use one dimensional sensitivity profiles of RF coils to encode space in a MRI context also have been proposed. For example, Ra, et al. (xe2x80x9cFast Imaging Using Sub-encoding Data Sets From Multiple Detectorsxe2x80x9d, Magn. Reson. Med., 30:142, 1993, the disclosure of which is hereby incorporated by reference herein), describes a method that uses sets of equally spaced k-space lines from multiple receiver coils in a line, and combines them with the one dimensional sensitivity profile information to remove the aliasing that occurs due to undersampling. A four-fold decrease in the image acquisition time of a water phantom was postulated to be possible by using an array of four coils.
Nevertheless, no biological images were shown in this article. It is believed that this may be indicative of a possible lack of robustness of the alaising removal algorithm in practical situations.
A method called SMASH proposed by Sodickson et al. (xe2x80x9cSimultaneous Acquisition Of Spatial Harmonics (SMASH): Fast Imaging With Radio Frequency Coil Arraysxe2x80x9d, Magn. Reson. Med., 38:591-603, 1997, the disclosure of which is hereby incorporated by reference herein) has been found to be practical, and yielded good results. SMASH enhances imaging speed by using multiple RF receiver coils. More specifically, it uses linear combinations of the 1D sensitivity profiles of receiver coils (weighted so as to form sinusoidal harmonics) of a one dimensional array to generate all k-space lines from a small subset of collected magnetic resonance data.
The SMASH method, however, is somewhat limited. It has an inherent inflexibility in the choice of the imaging plane to be viewed. Also, it has a demonstrable limitation in depth penetration. Further, there is a practical, physical limit on the number of coils that can be placed along one direction in a magnetic resonance imaging apparatusxe2x80x94particularly if the coils are to be de-coupled from one another.
The present invention provides an apparatus and a method that significantly decrease both magnetic resonance data acquisition time, and image reconstruction time, in magnetic resonance imaging.
Also, the present invention provides an apparatus and a method wherein sets of magnetic resonance information are acquired simultaneously, in parallel, with one another, and the elements of those sets preferably are subsequently processed in parallel with one another to reconstruct images.
These, and other, features and advantages of the present invention constitute a generalization of, and an improvement upon, the SMASH apparatus and method. More specifically, the present invention contemplates the placement of an array of RF coils, comprising substantially any number of RF receiver coils, at least partially around an object of interest located in the imaging volume of a magnetic imaging device. The present invention also contemplates the provision of a so-called xe2x80x9cparallelxe2x80x9d imaging capability wherein the output image may be taken in any plane transverse to the imaging volume of the apparatus.
In the present invention, parallel encoding in MRI is achieved by using the sensitivity profiles of an array of RF receiver coils at least partially surrounding the object of interest. Given this fact, the equation describing the MR signal seen by the ith coil of a coil array surrounding the imaging volume of a magnetic resonance imaging device can be written as:
Si(t)=∫∫xcfx81(x,y).Wi(x,y).ejxcex3(Gxxt+Gyyxcfx84)dx.dy
Where Wi(x,y) represents the 2D sensitivity profile of the ith coil of the array, and xcfx81(x, y) is an image slice in a selected (x, y) plane. Then, taking the Fourier transform of that signal with respect to x yields:
Fi(x)=FT[Si(t)]=∫xcfx81(x,y).Wi(x,y).ejxcex3Gyxcfx84.dy
The latter equation represents a projection of the phase modulated image xcfx81(x,y) onto the x-axis. Further, this signal can be represented in discrete form by the following matrix product:
Fi(x)=xcexa3y[Wi(x,y).ejxcex3Gyxcfx84].[xcfx81(x,y)]
If the number of receiver coils used simultaneously is N, and the 2D sensitivity profile Wi(x,y) of each one of them is known, it is possible to reconstruct the image from only one Nth of the total number of k-space lines that would normally be required. Accordingly, the apparatus and method of the present invention use phase modulated projections of the received magnetic resonance data onto the frequency encoded (x-) axis, weighted by the 2D sensitivity profiles of the coils in the array, in order to reconstruct xcfx81(x,y) column by column (i.e., orthogonal to the x-axis).
The 2D sensitivity profiles are calculated first by acquiring a baseline image using the RF body coil of the conventional magnetic resonance imaging apparatus, whereby the sensitivity profile may be considered to be constant (WB(x,y)=1) and a slice of the image in the (x, y) plane may be written as: xcfx81(x,y). An image is then acquired with each coil in the array. In the ith coil, this image can be written as:
xe2x80x83Wi(x,y)xcfx81(x,y)
Wi(x,y) is then computed by forming a point-by-point ratio between magnetic resonance data from the ith coil and that from the body coil.
It, therefore, will be seen that a preferred embodiment of the invention is a method for generating a magnetic resonance image of an object of interest composed of a plurality of adjacent image lines. The method generally includes the following steps. First, a magnetic resonance imaging device is provided. This device typically includes a magnet system providing a background magnetic field in an imaging volume, a central processor, a memory device, a RF coil surrounding the imaging volume, and a plurality of radio frequency receiver coils defining a multi-dimensional array thereof disposed about the imaging volume. A two-dimensional sensitivity profile for each receiver coil in the multi-dimensional array is then computed and recorded to the memory device. Thereafter, a plurality of magnetic resonance signals of the object of interest located within the imaging volume is acquired from each receiver coil and recorded to the memory device. An image of the object of interest in a desired plane extending transversely through the imaging volume of the magnetic resonance imaging device then is reconstructed line-by line by the central processor. This reconstruction combines the inverse of the matrix of the sensitivity profiles of each receiver coil and the matrix of the recorded MR data signals together to provide an image that may be displayed or printed.
Preferably, the method of the invention involves a calibration step wherein a homogeneous water phantom is provided to obtain the sensitivity profile of each receiver in the array. The homogeneous water phantom is located in the image volume of the magnetic resonance imaging device. Magnetic resonance data from the homogeneous water phantom in each plane of interest extending transversely through the imaging volume of the magnetic resonance imaging device then are acquired by a RF body coil and by each of the RF image receiver coils. This magnetic resonance data is stored in the memory of the magnetic resonance imaging device. Thereafter, the central processor calculates, and stores in the memory device, the respective point-by-point ratios of the complex data representing the image from the water phantom provided by said receiver coils to the complex data representing the image from the water phantom provided by the RF body coil.
In a preferred embodiment of the invention, an object of interest is located in the imaging volume of the magnetic resonance imaging device. Magnetic resonance data from the plane of interest is acquired from each of the RF receiver coils and stored in memory. Then, the respective point-by-point ratios of the magnetic resonance data from the plane of interest provided by the receiver coils to the magnetic resonance data from the water phantom are calculated so as to define a scaling factor for each point in the plane of interest. This scaling factor may then be utilized to provide a load-weighted, point-by-point sensitivity profile for each RF receiver coil in each plane of interest parallel to that for which the scaling factor was originally determined.
In still another embodiment of the invention, means are provided for determining and storing in memory the point-by-point time difference of signal reception by each of the receiver coils. This information may be combined with the sensitivity information in the inverse of the matrix of the sensitivity profile so as to provide a phase compensated, load-weighted, point-by-point, inverse, sensitivity profile matrix for each said receiver coil.
Further, it will be understood that the present invention includes apparatus for carrying out each of the above-described methods. Accordingly, the present invention also includes an apparatus for generating a magnetic resonance image of an object of interestxe2x80x94the image being composed of a plurality of adjacent image lines. More specifically, the apparatus preferably constitutes a magnetic resonance imaging device including an imaging volume, preprogrammed central processor, memory device, a body coil surrounding said imaging volume, a plurality of radio frequency receiver coils defining a multi-dimensional array thereof disposed about said imaging volume, and a display device. Each receiver coil in the multi-dimensional array has an associated two-dimensional sensitivity profile. Preferably, the preprogrammed central processing is adapted to calculate each of the sensitivity profiles and to transfer the same to the memory device.
The receiver coils are adapted to acquire simultaneously a plurality of magnetic resonance signals from an object of interest located within the imaging volume, and to transfer the same to the memory device. In addition, the central processor is adapted to reconstruct, and to display on the display device, line-by-line, a two-dimensional image taken in a selected plane extending transversely through the object of interest.
This is accomplished by multiplying the inverse of the matrix of sensitivity profiles of the receiver coils and the matrix of data signals acquired by the receiver coils, and displaying the resultant product on the display device.
In another embodiment, the magnetic resonance imaging device is adapted to acquire, and to transfer to the memory device, magnetic resonance data from a homogeneous phantom in each plane of interest extending through the imaging volume using said body coil. Similarly, the magnetic resonance imaging device also is adapted to acquire, and to transfer to the memory device, magnetic resonance data from a homogeneous water phantom in each plane of interest from each of said receiver coils. Also, the central processor is adapted to calculate, and transfer to the memory device, the respective point-by-point ratios of the magnetic resonance data from the water phantom provided by the receiver coils to the magnetic resonance data from the water phantom provided by the body coil. The component points of each image line may then be refined in the central processor by a corresponding one of these ratios prior to its display on the display device.
In yet another embodiment, the magnetic imaging device is adapted to acquire, and to transfer to the memory device, magnetic resonance data from the object of interest by each of the receiver coils. In this case, the central processor is adapted to calculate, and to transfer to the memory device, the respective point-by-point ratios of the magnetic resonance data from the object of interest provided by the receiver coils to the magnetic resonance data from the water phantom provided by the receiver coils. This ratio defines for each point a scaling factor. The central processor also is adapted to refine the component points of each image line by use of a corresponding one of the scaling factors prior to its display.
Finally, in still another embodiment of the invention, a phase detection device(s) is/are provided for determining and storing in the memory device the point-by-point time difference of signal reception by each of the receiver coils. In this alternative, the central processor is adapted to multiply the point-by-point time difference of signal reception by the point-by-point sensitivity profile for each said surface coil prior to the line-by-line image display on the display device.
These and other features, objects and advantages of the present invention, will be more completely understood by those skilled in the art from the following detailed description of the preferred embodiments of the invention read in conjunction with the drawings which follow.